Ke Chen, Tim Still, Kevin B. Aptowicz, Sam Schoenholz, Michael Schindler, A. C. Maggs, Andrea J. Liu, A. G. Yodh
The vibrational modes of monolayer colloidal crystals composed of thermosensitive microgel particles are measured using video microscopy and covariance matrix analysis. At low frequencies, the Debye relation for two dimensional harmonic crystals is observed, but at higher frequencies, van Hove singularities in the phonon density of states are significantly smeared out by experimental noise and measurement statistics. We introduce methods to correct for these errors, which can be applied to disordered systems as well as crystalline ones, and show that error correction leads to considerably more pronounced van Hove singularities. Finally, quasi-localized low-frequency modes in polycrystalline two-dimensional colloidal crystals are demonstrated to correlate with structural defects such as dislocations, suggesting that quasi-localized low-frequency phonon modes may be used to identify local regions vulnerable to rearrangements in crystalline as well as amorphous solids.
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http://arxiv.org/abs/1212.1741
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